1. Field of the Invention
The invention pertains to a test apparatus and method for assessing thermo-mechanical fatigue related phenomena within a test material, including but not limited to thermo-mechanically induced creep, residual stress, changes in physical properties, crack initiation and crack growth, occurring at temperatures below the melting temperature of the material.
2. Description of the Prior Art
Thermo-mechanical fatigue (TMF) testing is necessary in elevated temperature engineering applications where component durability and safety concerns merit the associated costs, which are often substantial, due to the difficulty of cycling both thermally and mechanically at the same time. A well-known type of TMF test is performed on a tubular specimen intermittently induction heated and cooled by air running through the tube, while simultaneously being loaded mechanically in strain control in a hydraulic test machine. A recent example of this class of testing is described in Thermal Mechanical Fatigue Cracks Growth from Laser Drilled Holes in Single Crystal Material, by Kersey et al (US Air Force Research Laboratory Report AFRL-RX-WP-TP-2012-0251, March 2012). Systems capable of this type of strain-controlled TMF testing are commercially available from well-known test equipment vendors, including specialized units that permit biaxial loading, and internationally recognized standard test methods exist. However, the high cost of the equipment, and the obvious complexities of the method can render it prohibitive.
Another class of TMF testing involves heating a specimen with a thermal gradient, which creates a corresponding passive mechanical stress gradient associated with mismatched thermal expansion. A simple example of an active-thermal, passive-mechanically cycled test used to assess a body of material (though it can also be coated) is described in U.S. Pat. No. 6,935,187 B1, which utilizes a specimen consisting of an elongated base with a fin extending along its length. When thermally cycled in an oven, the thermal lag in the base results in a cyclic thermal mismatch between the base and the fin, resulting in cyclic stresses, which can eventually result in cracks initiating and growing in the fin. While the method is vastly simpler than conventional strain-controlled TMF testing, thermocycling in an oven remains a substantial time and energy-consuming process. Also, the hot-compressive dwell time, an important parameter in TMF testing, is limited by the geometry, and cannot be varied at will because the thermal stresses vanish at steady state conditions.
Many such active-thermal, passive-mechanical test applications focus on the effect of TMF on surface integrity, especially as applied to coatings, including thermal barrier coatings (TBCs)—ceramic coatings used on parts like turbine blades to shield them from high temperature operational environments, and industrial hard coatings for tooling applications. These applications use various heat sources such as a torch, an infrared heater, or a laser beam to heat a surface to create a substantially through-thickness temperature distribution of largely planar geometry. That is, the isotherms in the region of interest are substantially planar, and parallel to each other. A summary of references of this kind are summarized in Table 1. Parameters not provided by authors were estimated or found in the literature. Values in the last three columns will be explained later on.
The relatively planar nature of the thermal distribution can be viewed as a result of one of two mechanisms. In the first, the test resembles a thin plate heated on one side and cooled on the other, naturally approaching a steady-state linear thermal distribution through the thickness, with the planar geometry described. The resulting differential temperature is sensitive to both the thermal flux and the thickness of the plate, and is also strongly influenced by the thermal resistance and variations in cooling rate at the cooling surface. Also, the planar nature of the thermal distribution requires that the size of the heated zone exceed the thickness of the specimen, typically requiring a high power heat source and high energy consumption during the test.
The second mechanism involves heating by heat pulses sufficiently short in duration that they penetrate a small depth into the surface relative to the size of the heated zone before they are substantially dissipated, thus also assuming a planar nature within the penetration depth. Peak temperatures reached are a function of heat flux magnitude and duration, and the thermal diffusivity of the material, but are insensitive to thickness and cooling method as the thickness becomes large compared to the penetration depth. Temperatures near the heated zone vary rapidly during the heating cycle and are of transient character, as this mechanism is not operable at near steady-state conditions.
TABLE 1Summary of Prior Art with Substantially Planar Thermal Distributions    Reference  Heat Source, Substrate MaterialThermal Diffusivity α mm2/sec (in2/sec)Heat Application Zone size d mm (in)  Heating Time th (sec)Radius of Fiducial Hemisphere rf mm (in)    d            αt      h               r    f    dM Aischeler, Thermal FatiguePulsed Laser,1111.264E-0896007.12Properties of Poycrystalline Copper inCopper(0.172)(0.050)(0.354)CLIC Accelerating Structures: SurfaceRoughness and Hardening as a Functionof Grain Orientation, 25th LinearAccelerator Conference, Tsubaka,Japan, 12-17 Sep., 2010.Bartosik et al, Lateral gradients ofContinuous-3563050.190.83phases, residual stress and hardness inWave laser,(0.054)(0.236)(0.197)a laser heated Ti0.52Al0.48N coating onWC-Cohard metal, Surface & CoatingsTechnology, vol. 206, 2012.Cote et al, Laser Pulse HeatingPulsed Laser,102.60.0052.5411.60.98Simulation of Firing Damage on CoatedSteel(0.016)(0.102)(0.100)Gun Bore Surfaces, Technical ReportARCCB-TR-01005, Benet Laboratories,2001.Zhu, Miller, Influence of High CyclePulsed & CW10320.0095.881070.18Thermal Loads on Thermal FatigueLaser,(0.016)(1.260)(0.231)Behavior of Thick Thermal BarrierSteel10323005.880.580.18Coatings, ARL-TR-1341, May 1997.(0.016)(1.260)(0.231)1032180014.70.240.46(0.016)(1.260)(0.579)Zhu et al, Oxidation- and Creep-Pulsed & CW4.832180010.340.03Enhanced Fatigue of Haynes 188 Alloy-Laser,(0.007)(1.260)(0.039)Oxide Scale System Under SimulatedHaynes 1884.8320.00114620.03Pulse Detonation Engine Conditions,(0.007)(1.260)(0.039)NASA TM-2002-211484, 2002.Nishinoiri et al, Evaluation ofInfra-red4101805.80.370.58Microfracture Mode in Ceramic CoatingImage(0.006)(0.394)(0.228)during Thermal Cycle Test using LaserFurnace, 304AE technique, Materials Transactions,SSvol. 45, no. 1, 2004.Panda et al, Thermal shock and thermalGas Torch,3.5132061.550.46fatigue study of ceramic materials on aSi3N4(0.005)(0.512)(0.236)newly developed ascending thermalshock test equipment, Science andTechnology of Advanced Materials,Vol. 3, 2002.Rymer, Stress Intensity Solutions ofGas Torch,1060251.53.790.03Thermally Induced Cracks in aB-1900 + Hf(0.016)(2.362)(0.059)Combustor Liner Hot Spot Using FiniteElement Analysis, Dissertation, GeorgiaInstitute of Technology, 2005.
While a more precise characterization of the operational regimes associated with “thin plate” and “short pulse” modes of operation described above will be given hereafter, a summary of the operational characteristics of these modes is given in Table 2, compared with a “desired state” which identifies some of the objects of the invention to be presented.
TABLE 2Operability SummaryOperableSensitive toEnergynearInsensitiveInsensitivethermal fluxCon-Mode ofsteadytoto coolingat heatedsump-Operationstatethickness*method*surfacetionThin PlateYesNoNoYesHighShort PulseNoYesYesYesLowDesired StateYesYesYesYesLow*A minimum thickness may be required to achieve this property.
While in some respects, a planar thermal distribution might seem to make sense for TMF testing of thin coatings—in concept, the thin plate linear thermal distribution is very simple to analyze, and matches the symmetry of a planar coating layer—limitations, such as those identified in Table. 2 can be significant in practice. As alluded to earlier, it is useful to be able to specify a desired thermo-mechanical cycle, often including a dwell time at constant temperature and applied mechanical strain, and then configure the test to approximate that cycle. This would require near-steady-state operability, which is only available in thin plate mode. It is also preferable for the time required to reach near-steady-state conditions to be small compared to the desired dwell times, which may not always be practical with thin plates for short dwell times.
Insensitivity to thickness allows reliable comparison between different specimen configurations, and is often accompanied by insensitivity to cooling methods, but in current practice is only available in the short pulse mode. Panda et al observed a very strong thickness effect in their thin plate data, despite mounting their specimen with a thermally conductive paste to a water-cooled copper pedestal to make sure their cooling was even and reproducible. Bartosik et al used a copper pedestal immersed in a thermally regulated liquid reservoir to control the temperature of their thin plate sample.
In addition to the specimen configurations tested by Zhu et al (2002) listed in Table 1, they apparently took advantage of the thickness independence of the short pulse mode to apply their method to structural components and conventional 4-point bend specimens. They did not give sufficient details to include the component and 4-point bend tests in the table, but they were run with a 0.8 ms heat pulse on various items. It is apparent that they could not run thickness-independent tests with dwell in this manner, thus their dwell tests were run on thin-plate specimens (and thin-plate-like 90 degree angles). It is also worth noting that to obtain the large spot size used (apparently motivated by their thin plate work), they employed a 1500W laser. Use of a less expensive and power consuming device would be desirable.
While most of the laser beams used in the prior art of Table 1 were nominally circular, Aicheller employed a rectangular beam. Cote et al used a fiber-optic cable to transmit the laser beam to the specimen, resulting in a more uniform flux distribution.
Also of interest in the prior art, theoretical thermal and thermo-elastic solutions for numerous potentially relevant heating scenarios are available in the literature. An application of one such solution to the problem of laser rock spallation is described by Xu et al in Laser Spallation of Rocks for Oil Well Drilling (Proceedings of the 23rd International Congress on Applications of Lasers and Electro-Optics, 2004).
Recently, a creep-shakedown-based TMF crack growth prediction approach has been a topic of study for analysis of the hot-compressive dwell cycle that typically results from active-thermal passive-mechanical TMF cycles, as described in a presentation entitled Effects of Hot Compressive Dwell Condition on Fatigue Crack Growth Response of Cast Aluminum Alloys by Xiang Chen et al of the Integrative Material Design Center (IMDC) at Worcester Polytechnic Institute (WPI) (presented to TMS 2012, 141st Meeting and Exhibition, Mar. 11-15, 2012, Orlando, Fla.). The method was applied to an aluminum casting material in the context of a cylinder head TMF problem. While the material characterization and analysis method can account for quite general hot-compressive time-dependent behavior in a variable-temperature environment, the partial validation performed utilized an isothermal test method that, while capturing the isothermal creep-related phenomena, could not rule out the possibility of additional non-isothermal effects that might further accelerate the crack. Non-isothermal testing was not performed because WPI lacked the resources to do so, a situation that could be remedied at many academic and industrial institutions if a more accessible means of TMF testing were available. This also suggests that, using the IMDC approach, a creep shakedown model for a given material could be developed and partially validated using simple isothermal testing, with a final validation involving a few non-isothermal TMF tests to prove whether the method properly predicts the more general behavior. Among other applications, the invention now to be discussed could fill this need.